Breast cancer is by far the most frequently diagnosed cancer and cause of cancer death among women. There were an estimated 1.7 million new cases (25% of all cancers in women) and 0.5 million cancer deaths (15% of all cancer deaths in women) in 2012. The annual incidence in industrialized countries is 70-90 new cases per 100000 women which is 2-fold higher compared to countries categorized as having low levels of development.
Age-standardized incidence rates are highest in western Europe and lowest in East Asia. Mortality rates vary approximately 2-5-fold worldwide; the case fatality rate is lower in countries with higher levels of human development. About 43% of the estimated new cases and 34% of the cancer deaths occurred in Europe and North America.
Whereas incidence has been generally increasing in most areas of the world, it has peaked and declined over the past decade in a number of highly developed countries. Mortality rates have been declining in a number of highly developed countries since the late 1980s and early 1990s, a result of a combination of improved detection and earlier diagnosis (through population-based screening) and more effective treatment regimens (World Cancer Report, 2014).
Well-characterized breast cancer risk factors include age, family history, reproductive factors including nulliparity, first birth after age 30, mammographic density, and atypia in a prior benign breast biopsy. Agents that cause breast cancer include alcohol consumption, use of combined estrogen-progestogen contraceptives and menopausal therapy, exposure to X- and γ-radiation and lifestyle factors such as high-calorie diets and lack of exercise. Physical activity has been associated with a 25-30% decrease in breast cancer risk due to a decrease of endogenous estrogens, adiposity, insulin resistance, leptin, and inflammation, which are all associated independently with increased breast cancer risk (Winzer et al., 2011).
A small proportion of breast cancers are due to inherited mutations in high-penetrance breast cancer susceptibility genes (BRCA1 and BRCA2).
Several lower-penetrance genes have been discovered by next-generation sequencing technology applied to examine the genomes of 100 breast cancers for somatic copy number changes and mutations in the coding exons of protein-coding genes (Stephens et al., 2012). The number of somatic mutations varied markedly between individual tumors. Strong correlations were evident between number of mutations, age at which cancer was diagnosed, and cancer histological grade, and multiple mutational signatures were observed, including one, present in about 10% of tumors, characterized by numerous mutations of cytosine at TpC dinucleotides. Driver mutations were identified in several new cancer genes, including AKT2, ARID1B, CASP8, CDKN1B, MAP3K1, MAP3K13, NCOR1, SMARCD1, and TBX3. Among the 100 tumors, there were driver mutations in at least 40 cancer genes and 73 different combinations of mutated cancer genes. Overall, one of the most highly contributing genes to development of breast cancer, TP53, is nonetheless probably only involved in 25% of cancers, although its role in subsets of breast cancers, such as triple-negative/basal-like, is much higher. A large number of genes appear to be implicated in a small percentage of tumors, and this will provide additional challenges to achieving the dream of a patient-specific targeted and personalized medical intervention.
Breast cancer is not a single disease and is heterogeneous both clinically and morphologically. The current WHO Classification of Tumors of the Breast (4th edition) recognizes more than 20 different subtypes. Most breast cancers arise from epithelial cells (carcinomas); these tumors are subdivided into in situ and invasive lesions. In situ carcinomas are preinvasive lesions and further subdivided into ductal carcinoma in situ (DCIS) and lobular carcinoma in situ (LCIS). The distinction between DCIS and LCIS is the result not of the micro-anatomical site of origin (ducts vs lobules) but rather of a difference in the architectural and cytological features of the cells. DCIS and LCIS also differ in their distribution within the breast, as well as their risk of recurrence and progression to invasive cancer.
Invasive carcinoma characterized as “no special type”, also known as ductal carcinoma no special type or invasive ductal carcinoma, makes up the largest subset of invasive breast cancer. This designation identifies a heterogeneous group comprising tumors that are not easily categorized by specific morphological features that characterize the “special subtypes”. Hence, it is a default diagnosis for all those tumors (approximately 70%) that cannot be assigned a “special subtype” designation. The most common of the special subtypes include lobular carcinoma, tubular carcinoma, mucinous carcinoma, carcinoma with medullary and apocrine features, micropapillary and papillary carcinomas, and metaplastic carcinomas.
Histological grade is a measure of how closely a tumor resembles its tissue of origin and is an integral part of a pathology report. The current grading system assesses the three parameters degree of architectural differentiation, nuclear pleomorphism and proliferation of the tumor. Although this semiquantitative approach averages the intratumor heterogeneity that exists in many tumors, it remains a powerful indicator of patient prognosis. Histological grade is also strongly associated with histological type and with patterns of molecular alterations, such as estrogen receptor (ER) and progesterone receptor (PR) expression and human epidermal growth factor receptor 2 (HER2) protein over-expression and gene amplification.
Recent molecular and genetic studies have emphasized that breast cancer is a highly heterogeneous group of diseases that differ in their prognosis and response to treatment. Improved understanding of the molecular pathways and genetic alterations that underlie the different breast cancer subtypes is leading to a more targeted and personalized approach to breast cancer treatment.
The standard treatment for breast cancer patients depends on different parameters: tumor stage, hormone receptor status and HER2 expression pattern. The standard of care includes complete surgical resection of the tumor followed by radiation therapy. Chemotherapy with mainly anthracyclines and taxanes may be started prior to or after resection. In advanced stages, additional chemotherapeutics like alkylating agents, fluorpyrimidines, platinum analogs, etc. can be applied as mono- or combination therapy. Patients with HER2-positive tumors receive the anti-HER2 antibody trastuzumab in addition to the chemotherapeutics. The vascular endothelial growth factor (VEGF) inhibitor bevacizumab synergizes with paclitaxel or capecitabine monotherapy. Chemotherapy is typically less efficient in patients with estrogen or progesterone receptorpositive tumors. For these patients, the standard regimen comprises an endocrine therapy with tamoxifen (first line) or aromatase inhibitors (second line) after initial chemotherapy (S3-Leitlinie Mammakarzinom, 2012).
For more than a decade, gene expression profiling has been applied to define molecular phenotypes of breast cancer and luminal A and basal-like subtypes were found the two main subtypes of five that have been identified. This type of analysis has not only confirmed the two large subsets of breast cancer—ER-positive and ER-negative—but also brought to the fore differences within the ER categories.
Using these gene expression data, it is more and more possible to classify breast cancer, develop signatures for “good” versus “poor” prognosis, and identify tumors that may or may not respond to particular therapy (van de Vijver et al., 2002). Rapidly evolving knowledge of the molecular events and signaling pathways underlying the development and progression of breast cancer has also led to the identification of a growing number of new therapeutic targets and to the development of drugs against these targets. Many clinical trials are currently under way worldwide to evaluate the role of these new targeted therapies in the treatment of patients with breast cancer. Newer targeted therapies that have been or are being evaluated, singly and in combination with each other and with traditional cytotoxic agents, include angiogenesis inhibitors, tyrosine kinase inhibitors, inhibitors of mammalian target of rapamycin (mTOR), poly(ADP-ribose) polymerase 1 (PARP1) inhibitors, insulin-like growth factor 1 receptor (IGF-1R) inhibitors, proteasome inhibitors, phosphatidylinositol 3-kinase (PI3K) inhibitors, and others (Perez and Spano, 2012). The ultimate goal of this research is to be able to tailor breast cancer treatment for individual patients based on the particular molecular features of the tumor. The molecular analyses are also relevant to survival. One study used modelling of messenger RNA (mRNA), copy number alterations, microRNAs, and methylation (Kristensen et al., 2012). For all breast cancers, the strongest predictor of good outcome was acquisition of a gene signature that favored a high T helper type 1 (Th1)/cytotoxic T lymphocyte response at the expense of Th2-driven immunity.
These data reflect that breast cancer is an immunogenic cancer entity and different types of infiltrating immune cells in primary tumors exhibit distinct prognostic and predictive significance. A large number of early phase immunotherapy trials have been conducted in breast cancer patients. Most of the completed vaccination studies targeted HER2 and carbohydrate antigens like MUC-1 and revealed rather disappointing results. Clinical data on the effects of immune checkpoint modulation with ipilimumab and other T cell-activating antibodies in breast cancer patients are emerging (Emens, 2012).
Considering the severe side-effects and expense associated with treating cancer, there is a need to identify factors that can be used in the treatment of cancer in general and breast cancer in particular. There is also a need to identify factors representing biomarkers for cancer in general and breast cancer in particular, leading to better diagnosis of cancer, assessment of prognosis, and prediction of treatment success.
Immunotherapy of cancer represents an option of specific targeting of cancer cells while minimizing side effects. Cancer immunotherapy makes use of the existence of tumor associated antigens.
The current classification of tumor associated antigens (TAAs) comprises the following major groups:
a) Cancer-testis antigens: The first TAAs ever identified that can be recognized by T cells belong to this class, which was originally called cancer-testis (CT) antigens because of the expression of its members in histologically different human tumors and, among normal tissues, only in spermatocytes/spermatogonia of testis and, occasionally, in placenta. Since the cells of testis do not express class I and II HLA molecules, these antigens cannot be recognized by T cells in normal tissues and can therefore be considered as immunologically tumor-specific. Well-known examples for CT antigens are the MAGE family members and NY-ESO-1.
b) Differentiation antigens: These TAAs are shared between tumors and the normal tissue from which the tumor arose. Most of the known differentiation antigens are found in melanomas and normal melanocytes. Many of these melanocyte lineage-related proteins are involved in biosynthesis of melanin and are therefore not tumor specific but nevertheless are widely used for cancer immunotherapy. Examples include, but are not limited to, tyrosinase and Melan-A/MART-1 for melanoma or PSA for prostate cancer.
c) Over-expressed TAAs: Genes encoding widely expressed TAAs have been detected in histologically different types of tumors as well as in many normal tissues, generally with lower expression levels. It is possible that many of the epitopes processed and potentially presented by normal tissues are below the threshold level for T-cell recognition, while their over-expression in tumor cells can trigger an anticancer response by breaking previously established tolerance. Prominent examples for this class of TAAs are Her-2/neu, survivin, telomerase, or WT1.
d) Tumor-specific antigens: These unique TAAs arise from mutations of normal genes (such as β-catenin, CDK4, etc.). Some of these molecular changes are associated with neoplastic transformation and/or progression. Tumor-specific antigens are generally able to induce strong immune responses without bearing the risk for autoimmune reactions against normal tissues. On the other hand, these TAAs are in most cases only relevant to the exact tumor on which they were identified and are usually not shared between many individual tumors. Tumor-specificity (or -association) of a peptide may also arise if the peptide originates from a tumor-(-associated) exon in case of proteins with tumor-specific (-associated) isoforms.
e) TAAs arising from abnormal post-translational modifications: Such TAAs may arise from proteins which are neither specific nor overexpressed in tumors but nevertheless become tumor associated by posttranslational processes primarily active in tumors. Examples for this class arise from altered glycosylation patterns leading to novel epitopes in tumors as for MUC1 or events like protein splicing during degradation which may or may not be tumor specific.
f) Oncoviral proteins: These TAAs are viral proteins that may play a critical role in the oncogenic process and, because they are foreign (not of human origin), they can evoke a T-cell response. Examples of such proteins are the human papilloma type 16 virus proteins, E6 and E7, which are expressed in cervical carcinoma.
T-cell based immunotherapy targets peptide epitopes derived from tumor-associated or tumor-specific proteins, which are presented by molecules of the major histocompatibility complex (MHC). The antigens that are recognized by the tumor specific T lymphocytes, that is, the epitopes thereof, can be molecules derived from all protein classes, such as enzymes, receptors, transcription factors, etc. which are expressed and, as compared to unaltered cells of the same origin, usually up-regulated in cells of the respective tumor.
There are two classes of MHC-molecules, MHC class I and MHC class II. MHC class I molecules are composed of an alpha heavy chain and beta-2-microglobulin, MHC class II molecules of an alpha and a beta chain. Their three-dimensional conformation results in a binding groove, which is used for non-covalent interaction with peptides.
MHC class I molecules can be found on most nucleated cells. They present peptides that result from proteolytic cleavage of predominantly endogenous proteins, defective ribosomal products (DRIPs) and larger peptides. However, peptides derived from endosomal compartments or exogenous sources are also frequently found on MHC class I molecules. This non-classical way of class I presentation is referred to as crosspresentation in the literature (Brossart and Bevan, 1997; Rock et al., 1990). MHC class II molecules can be found predominantly on professional antigen presenting cells (APCs), and primarily present peptides of exogenous or transmembrane proteins that are taken up by APCs e.g. during endocytosis, and are subsequently processed.
Complexes of peptide and MHC class I are recognized by CD8-positive T cells bearing the appropriate T-cell receptor (TCR), whereas complexes of peptide and MHC class II molecules are recognized by CD4-positive-helper-T cells bearing the appropriate TCR. It is well known that the TCR, the peptide and the MHC are thereby present in a stoichiometric amount of 1:1:1.
CD4-positive helper T cells play an important role in inducing and sustaining effective responses by CD8-positive cytotoxic T cells. The identification of CD4-positive T-cell epitopes derived from tumor associated antigens (TAA) is of great importance for the development of pharmaceutical products for triggering anti-tumor immune responses (Gnjatic et al., 2003). At the tumor site, T helper cells, support a cytotoxic T cell-(CTL-) friendly cytokine milieu (Mortara et al., 2006) and attract effector cells, e.g. CTLs, natural killer (NK) cells, macrophages, and granulocytes (Hwang et al., 2007).
In the absence of inflammation, expression of MHC class II molecules is mainly restricted to cells of the immune system, especially professional antigen-presenting cells (APC), e.g., monocytes, monocyte-derived cells, macrophages, dendritic cells. In cancer patients, cells of the tumor have been found to express MHC class II molecules (Dengjel et al., 2006).
Elongated (longer) peptides of the invention can act as MHC class II active epitopes. T-helper cells, activated by MHC class II epitopes, play an important role in orchestrating the effector function of CTLs in anti-tumor immunity. T-helper cell epitopes that trigger a T-helper cell response of the TH1 type support effector functions of CD8-positive killer T cells, which include cytotoxic functions directed against tumor cells displaying tumor-associated peptide/MHC complexes on their cell surfaces. In this way tumor-associated T-helper cell peptide epitopes, alone or in combination with other tumor-associated peptides, can serve as active pharmaceutical ingredients of vaccine compositions that stimulate anti-tumor immune responses.
It was shown in mammalian animal models, e.g., mice, that even in the absence of CD8-positive T lymphocytes, CD4-positive T cells are sufficient for inhibiting manifestation of tumors via inhibition of angiogenesis by secretion of interferon-gamma (IFNγ) (Beatty and Paterson, 2001; Mumberg et al., 1999). There is evidence for CD4 T cells as direct anti-tumor effectors (Braumuller et al., 2013; Tran et al., 2014).
Since the constitutive expression of HLA class II molecules is usually limited to immune cells, the possibility of isolating class II peptides directly from primary tumors was previously not considered possible. However, Dengjel et al. were successful in identifying a number of MHC Class II epitopes directly from tumors (WO 2007/028574, EP 1 760 088 B1).
Since both types of response, CD8 and CD4 dependent, contribute jointly and synergistically to the anti-tumor effect, the identification and characterization of tumor-associated antigens recognized by either CD8+ T cells (ligand: MHC class I molecule+peptide epitope) or by CD4-positive T-helper cells (ligand: MHC class II molecule+peptide epitope) is important in the development of tumor vaccines.
For an MHC class I peptide to trigger (elicit) a cellular immune response, it also must bind to an MHC-molecule. This process is dependent on the allele of the MHC-molecule and specific polymorphisms of the amino acid sequence of the peptide. MHC-class-I-binding peptides are usually 8-12 amino acid residues in length and usually contain two conserved residues (“anchors”) in their sequence that interact with the corresponding binding groove of the MHC-molecule. In this way, each MHC allele has a “binding motif” determining which peptides can bind specifically to the binding groove.
In the MHC class I dependent immune reaction, peptides not only have to be able to bind to certain MHC class I molecules expressed by tumor cells, they subsequently also have to be recognized by T cells bearing specific T cell receptors (TCR).
For proteins to be recognized by T-lymphocytes as tumor-specific or -associated antigens, and to be used in a therapy, particular prerequisites must be fulfilled. The antigen should be expressed mainly by tumor cells and not, or in comparably small amounts, by normal healthy tissues. In a preferred embodiment, the peptide should be over-presented by tumor cells as compared to normal healthy tissues. It is furthermore desirable that the respective antigen is not only present in a type of tumor, but also in high concentrations (i.e. copy numbers of the respective peptide per cell). Tumor-specific and tumor-associated antigens are often derived from proteins directly involved in transformation of a normal cell to a tumor cell due to their function, e.g. in cell cycle control or suppression of apoptosis. Additionally, downstream targets of the proteins directly causative for a transformation may be up-regulated and thus may be indirectly tumor-associated. Such indirect tumor-associated antigens may also be targets of a vaccination approach (Singh-Jasuja et al., 2004). It is essential that epitopes are present in the amino acid sequence of the antigen, in order to ensure that such a peptide (“immunogenic peptide”), being derived from a tumor associated antigen, leads to an in vitro or in vivo T-cell-response.
Basically, any peptide able to bind an MHC molecule may function as a T-cell epitope. A prerequisite for the induction of an in vitro or in vivo T-cell-response is the presence of a T cell having a corresponding TCR and the absence of immunological tolerance for this particular epitope.
Therefore, TAAs are a starting point for the development of a T cell based therapy including but not limited to tumor vaccines. The methods for identifying and characterizing the TAAs are usually based on the use of T-cells that can be isolated from patients or healthy subjects, or they are based on the generation of differential transcription profiles or differential peptide expression patterns between tumors and normal tissues.
However, the identification of genes over-expressed in tumor tissues or human tumor cell lines, or selectively expressed in such tissues or cell lines, does not provide precise information as to the use of the antigens being transcribed from these genes in an immune therapy. This is because only an individual subpopulation of epitopes of these antigens are suitable for such an application since a T cell with a corresponding TCR has to be present and the immunological tolerance for this particular epitope needs to be absent or minimal. In a very preferred embodiment of the invention it is therefore important to select only those over- or selectively presented peptides against which a functional and/or a proliferating T cell can be found. Such a functional T cell is defined as a T cell, which upon stimulation with a specific antigen can be clonally expanded and is able to execute effector functions (“effector T cell”).
In case of targeting peptide-MHC by specific TCRs (e.g. soluble TCRs) and antibodies or other binding molecules (scaffolds) according to the invention, the immunogenicity of the underlying peptides is secondary. In these cases, the presentation is the determining factor.